Metal oxide (MO) semiconductors have attracted considerable attention for next-generation electronic devices because of their high carrier mobilities and good environmental stability. In addition, the high optical transparency of MO semiconductors could enable fully transparent thin-film transistors (TFTs), which are essential for the fabrication of “invisible” circuits and to increase the aperture ratio of active-matrix organic light-emitting diode (AMOLED) and liquid-crystal (LC) displays. Therefore, since the first report of a fully transparent MO-based TFT in 2003, extensive academic and industrial efforts have focused on enhancing device performance for both opaque and transparent applications. Nevertheless, the best-performing MO TFTs are typically fabricated by capital-intensive physical and chemical vapor deposition processes such as sputtering, and patterned using multi-step photolithographic processes. To enable inexpensive large-scale roll-to-roll production, it is necessary to develop solution-based process methodologies for the manufacturing of MO TFTs.
Another feature desired by next-generation electronic devices is mechanical flexibility. It is well known that polycrystalline films have limited mechanical flexibility, and mainly due to crack formation at grain boundaries, their electrical properties and structural integrity tend to deteriorate dramatically upon bending. Compared to polycrystalline MOs, amorphous MO semiconductors are more tolerant to mechanical stress, enabling their utilization and device fabrication on flexible substrates.
A well-known strategy to produce semiconducting amorphous MOs is to dope polycrystalline materials such as indium oxide (In2O3) with various X cations (e.g., X=Ga3+, Zn2+, La3+, Sc3+) to form ternary or quaternary amorphous alloys of formula IXO and IXZO composites (Z=zinc). An example of a technologically relevant amorphous IXZO material is indium-gallium-zinc-oxide (IGZO), which has excellent charge transport uniformity due to minimal structural defects. However, the carrier mobilities of these amorphous oxides are limited compared to that of the pristine In2O3 matrix. Efficient transport in In2O3 mainly originates from the diffuse In 5s orbitals at the bottom of the conduction band, leading to edge-sharing In—O6 octahedra. In contrast, the oxygen vacancies regulating the carrier concentrations are difficult to control in In2O3, thus the resulting TFTs exhibit less than optimum current modulation (Ion/Ioff) and poor threshold voltage (VT) uniformity over large areas. Furthermore, solution-processed amorphous IXO- and IXZO-based TFTs not only exhibit lower electron mobilities than In2O3, but also require relative higher processing temperatures (typically ≧300° C.) to facilitate metal-oxygen-metal (M-O-M) lattice formation, densification, and impurity removal. Such high processing temperatures are incompatible with inexpensive plastic substrates.
Similar limitations apply to conducting metal oxides where the best electrical conductivities are typically achieved for polycrystalline films such as tin-doped indium oxide (ITO). However, the electrical conductivity of ITO-coated plastic degrades severely upon multiple bending because of crack formation. Furthermore, although most of the MO dielectric materials are amorphous, the mechanical flexibility of these films tend to be limited because they are typically far thicker than the conducting and semiconducting layers used in the TFT stack.
Accordingly, there is a need in the art for semiconducting and conducting metal oxide films and electronic devices that can be processed at low temperatures, yet exhibiting charge transport characteristics that are comparable to polycrystalline MO semiconductor-based devices and yet having mechanical flexibility typical of those based on amorphous semiconducting films.